Plumbing Questions? Ask a Plumber and Get Answers ASAP.
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What is the model number of your unit?
Assuming you have adequate flow to fire up the unit this is probably a problem with the water flow sensor or the water flow servo. Do you have the manual? do you have an ohm meter? Could also be due to a cross over. Is there a shut off on the hot water line out of the unit?
There isn't much a homeowner can do aside from cleaning the inlet filter. There is one thing you can try but it may not be a good idea if you aren't familiar with this unit. Is the display blank and is there clicking coming from the unit?
Ok, then there's nothing you further you can do. You'll have to call a plumber who has Rinnai service training
Glad you worked it out. Sounds like you didn't have adequate flow to fire it up.